1. Field of the Invention
This invention relates generally to compressors and more particularly to a compressor that can be easily adapted for use in any one of multiple systems so that identical compressor modules can be manufactured but can be installed in different applications having different operating conditions. The compressor can also be used in an application which has multiple operating conditions, such as a refrigeration system having multiple compartments maintained at multiple different temperatures, and can automatically change its operating mode in response to changes in operating conditions.
2. Description of the Related Art
Refrigeration and air conditioning systems have long used the phase change refrigeration cycle for cooling objects, such as food, to reduced temperatures and for maintaining them at a reduced temperature for preservation. These refrigeration systems have a closed loop in which a refrigerant is compressed to a compressor discharge pressure and discharged into a heat exchanging condenser where heat is rejected. Refrigerant is accumulated in the condenser and metered from the condenser into an evaporator where heat is accepted through the evaporator walls from the stored objects in thermal connection with the evaporator. The rate at which the refrigerant is metered into the evaporator is controlled by a temperature control system which senses the temperature of the objects and meters the refrigerant into the evaporator as a function of evaporator temperature. From the evaporator the refrigerant is returned at a suction pressure to a compressor to complete the closed loop.
The design temperature that is sought to be maintained by a refrigeration system is dependent upon the particular application. Because there are an extensive variety of applications for refrigeration systems, their design temperatures vary over a wide range. For example, a refrigerator may be designed to preserve food in a state that does not damage some foods or require thawing of the foods. This refrigerator has a fresh food compartment in which food is retained in a relatively higher temperature zone at slightly above 0° C. Another refrigeration system may be designed to freeze food and therefore has a freezer compartment in which food is retained in a significantly lower temperature zone, for example in the range of −17° C. to −40° C. In the higher temperature zone of the fresh food compartment, the refrigerant is exposed to a higher temperature than in the lower temperature freezer zone. Consequently, the refrigerant in the evaporator of the fresh food compartment has a higher temperature, pressure and density and therefore presents a higher suction pressure to the compressor. The refrigerant in the evaporator of the freezer compartment is exposed to a lower temperature and therefore has a lower temperature, pressure and density and therefore presents the compressor with a lower suction pressure.
Alternatively, a single refrigeration system can be provided with multiple cooling compartments for maintaining objects at a different temperature in each compartment. A refrigerator may have both a fresh food compartment and a freezer compartment in which food is retained at the above temperatures. In the higher temperature zone of the fresh food compartment, the refrigerant is exposed to a higher temperature than in the lower temperature freezer zone. Consequently, the refrigerant in the evaporator of the fresh food compartment has a higher temperature, pressure and density and therefore presents a higher suction pressure to the compressor. The refrigerant in the evaporator of the freezer compartment is exposed to a lower temperature and therefore has a lower temperature, pressure and density and therefore presents the compressor with a lower suction pressure. Although it is common to provide home refrigerators with only a single evaporator and it is also known to connect two evaporators in series, a compressor can supply refrigerant to multiple evaporators for multiple compartments, with each compartment having its own temperature control system and expansion valve for metering refrigerant flow through the expansion valve for its compartment.
Although compressors are used in refrigeration systems, they are also used in a variety of other applications. Compressor applications expose compressors to different suction pressures and other intake gas parameters, either because compressors of one design may be installed in different applications each having a different operating condition or because a single compressor may be exposed to multiple operating conditions in a single application. The existence of these different operating conditions means that the compressor may alternatively be exposed to: (1) a relatively higher gas temperature, density and suction pressure and (2) a relatively lower gas temperature, density and suction pressure. For a compressor that has a piston reciprocating in a cylinder, the volume flow rate is proportional to the piston stroke and is simply the displacement. However, the mass flow rate is not only a function of the displacement, but also is a function of the gas density. This is the result of the fact that the same volume of a gas has less mass if the gas is at a lower density and more mass if at a higher density. Therefore, when a refrigeration compressor is presented with different operating conditions that provide a gas at different densities and suction pressures, the mass flow rate through the compressor changes in response to changes in density, temperature and suction pressure of the gas refrigerant.
If a compressor is optimized for any one of these multiple operating conditions, it will not be optimized for the other. If it is optimized for an intermediate condition, it will not be optimized for any. For example, if the compressor is designed to operate at a relatively lower suction pressure and density and to provide a selected mass flow rate under that operating condition, if it is operated at a relatively higher suction pressure and density condition, the mass flow rate through the compressor will increase approximately in proportion to the change in the density.
Under the condition of a higher temperature, density and suction pressure, such as for supplying refrigerant to cool a fresh food compartment, the compressor should have a lower compression ratio in order to deliver compressed refrigerant gas at the desired discharge pressure into the condenser. At the condition of a lower temperature, density and suction pressure, such as for supplying refrigerant for cooling a freezer compartment, a higher compression ratio is needed to deliver the gas to the condenser at the desired discharge pressure.
It is an object and feature of the invention to provide a compressor that can adapt to these different operating conditions by changing its mode of operation in order to optimize its efficiency and provide the desired compression ratio for both operating conditions.